That cross-linking of polymers may be accomplished chemically through utilization of chemical cross-linking agents is well known to those of skill in the art. For instance, cross-linking agents, such as organic peroxides, have been used to cross-link polyethylene polymers and copolymers. A general discussion of chemical cross-linking can be found at pages 331 to 414 of volume 4 of the Encyclopedia of Polymer Science and Technology, Plastics, Resins, Rubbers, Fibers published by John Wiley & Sons, Inc. and copyrighted in 1966. This document has a United States Library of Congress Catalog Card Number of 64-22188 and the referenced pages are hereby incorporated by reference. Typically, the chemical cross-linking agents react with the polymer to form a solid or highly viscous cross-linked copolymer. Furthermore, it is also known from U.S. Pat. No. 4,515,745 (1985) to Churms that when an organic peroxide chemical cross-linking agent is used in ethylene vinyl acetate (EVA) copolymer, in an amount small enough not to form gel measurable by ASTM test method number D 2765 (ASTM D2765 is further discussed below), i.e. the EVA containing a cross-linking agent is soluble in a solvent such as xylene, then the rheology during extrusion is modified, i.e. the bubble is more stable as the EVA is more easily stretchable. Similarly, U.S. Pat. No. 4,614,764 (1986) to Colombo et al shows greater bubble stability in the blown tubular film extrusion process by using organic peroxides, optionally in combination with an unsaturated silane, in linear low density polyethylene (LLDPE).
It is also generally well known in the art that irradiation, such as by electron beam irradiation, of certain polymeric film materials results in the irradiative cross-linking of the polymeric molecular chains contained therein and that such action generally results in a material having improved heat shrink properties, abuse resistance, structural integrity, tensile strength, puncture resistance, and/or delamination resistance. Such physical improvements from irradiation, in particular the improved heat shrink properties, are discussed in U.S. Pat. No. 3,022,543 (1962) to Baird et. al. Many of the other physical improvements also are discussed at columns 2 and 8 of U.S. Pat. No. 4,178,401 (1979) to Weinberg and at column 4 of U.S. Pat. No. 3,741,253 to Brax. Furthermore, it is also known from U.S. Pat. No. 4,525,257 (1985) to Kurtz et al that low level irradiation under 2 MR of narrow molecular weight distribution, linear low density ethylene/alpha-olefin (LLDPE) particulate copolymer sufficient to introduce cross-links into the particulate copolymer but insufficient to provide for significant measurable gelation produces improved copolymer rheology providing increased extensional viscosity during film fabrication, i.e. the bubble is more stable as the LLCPE is more easily stretchable.
Therefore, it would seem that combining a chemical cross-linking agent with irradiative cross-linking, even if both were at low levels, would have a cumulative effect whereby the large interconnected polymer chains from the amount of cross-linking from both in the polymer would result in such high polymer viscosity as to cause difficulty during stretching so that an oriented tube could not be blown. However, it has been unexpectedly discovered that the combination of a chemical cross-linking agent with irradiation has resulted in the orientation process for manufacturing polymeric films having a decreased time, i.e. increased speed or rate.